Lol people do not understand the airline industry. We keep thinking of one flight, one issue.
you have thousands and thousands of flights, with millions and millions of passengers per year. Statistically, a number of them will not pitch and its the job of the Revenue Management Systems, and related staff for churn etc to calculate how best the airline minimizes losses. And one of those ways is through overallocation of seats. Literally 95% of the time its working fine, as its based on real-world numbers. Sometimes though its wrong, and a passenger is affected in which case the airline must compensate accordingly.
Hotels do something similar, conference venues etc - its a fine art to maximize profit through knowin your churn, your rate of no-shows/cancellations, and how to find a balance between them.